Problem: In geometry class, the girl to boy ratio is $8$ to $5$. If there are a total of $39$ students, how many girls are there?
Answer: A ratio of $8$ girls to $5$ boys means that a set of $13$ students will have $8$ girls and $5$ boys. A class of $39$ students has $3$ sets of $13$ students. Because we know that there are $8$ girls in each set of $13$ students, the class must have $3$ groups of $8$ girls each. There is a total of $24$ girls in geometry class.